Given how frequently neoconservatives quote Winston Churchill in so many other contexts, it is striking that they don’t take to heart this particular admonition of his: “The Statesman who yields to war fever must realise that once the signal is given, he is no longer the master of policy but the slave of unforeseeable and uncontrollable events.” Former president Barack Obama took a lot of flak for winding down the U.S. military presence in Iraq in 2010. But given how hard it is to disengage from a major military commitment, the fact that he was able to do so at all is remarkable. The lingering U.S. commitment to Afghanistan, which President Trump now proposes to increase again, is more typical of the general pattern of easy in, not-so-easy out for major military commitments, even ones that are clearly not going to end well no matter how long we stay the course.

A THIRD lesson might be called the impotence of idle threats. Inasmuch as Richard Nixon had a “secret plan” for peace, as he claimed during the 1968 campaign, it was based on the so-called “madman theory,” a stratagem of persuading an enemy of one’s irrationality and then threatening the use of nuclear weapons to force a settlement. “They’ll believe any threat of force Nixon makes because it’s Nixon,” the candidate confidently told aide Bob Haldeman while strolling on the beach.

“We’ll just slip the word to them that, ‘for God’s sake, you know Nixon’s obsessed about Communism . . . and he has his hand on the nuclear button’—and Ho Chi Minh himself will be in Paris in two days begging for peace.”

Nixon’s madman approach drew upon the mistaken belief, an article of faith among some Republicans, that Eisenhower had forced the Chinese to settle in Korea in 1953 by making nuclear threats. It also derived from academic studies of “diplomatic blackmail” developed by the economist Thomas Schelling and, ironically, by Nixon’s future nemesis, Daniel Ellsberg.

Nixon’s madman diplomacy failed. Through various sources, he warned Hanoi in the summer and fall of 1969 that if a settlement were not reached by November 1 he would resort to “measures of great consequence and force.” His threats were singularly ill timed. Ho Chi Minh died in September, and the hard-liners who had long since supplanted him were not about to dishonor his cause by giving in to the United States. Those Nixon officials tasked to devise a military program to coerce North Vietnam concluded that nothing was likely to work, and that drastic escalation would bear a high domestic political cost.

It would be natural for anyone assuming the American presidency to be awed by our military power. It is human nature to believe that small nations, even ones with nuclear weapons, can be easily cowed by Mr. Big. In fact, as various presidents have discovered, there are numerous powerful internal and external constraints on the use of the nation’s vast power. And hard experience should have taught us by now that other nations and peoples, however small and seemingly weak, are not easily intimidated.

Another cautionary “lesson” from Vietnam is to resist the temptation of quick-fix strategies—in football lingo, the Hail Mary pass. In the run-up to massive U.S. involvement in Vietnam, decisionmakers like Assistant Secretary of Defense John McNaughton, already doubtful that military intervention would compel North Vietnam to stop aiding its southern allies, sought a Hail Mary pass in the theories of coercive bombing developed by Schelling, his friend and former Harvard colleague. In late 1964, working with the State Department, McNaughton devised a bombing campaign of graduated pressures against North Vietnam that he hoped would achieve U.S. goals and avert a disaster in Vietnam. “What we need is a theory that will limit our role,” he confided to his diary.

Schelling’s model and the Rolling Thunder air campaign that drew upon it erroneously assumed the same cost/benefit calculations on both sides. In fact, the United States sought merely to demonstrate its resolve to allies and enemies; North Vietnam sought to liberate what it considered its homeland. Rolling Thunder evolved into a prolonged, vastly destructive and ultimately futile exercise in coercion.

The Trump administration, similarly, faces a set of bad choices in Afghanistan. Setting aside his own well-founded instincts against escalation, the president was persuaded by his top advisers that a hasty withdrawal would create a vacuum for terrorists such as ISIS and Al Qaeda. Like McNaughton, and Trump’s predecessor, Barack Obama, he succumbed to the lure of the quick fix in the graveyard of empires. Hail Mary passes rarely win football games. Why should we expect them to win wars?

IN VIETNAM, both U.S. president Lyndon Johnson and his North Vietnamese counterpart, party general secretary Le Duan, repeatedly miscalculated how each would respond to the other’s initiatives. In escalating the war in 1965, Johnson and his advisers assumed—incorrectly—that North Vietnam would acquiesce to an independent South Vietnam rather than risk destruction at the hands of America’s vast military power. In 1964, and again in 1968 and 1972, the aggressive—and reckless—Le Duan assumed that he could win in South Vietnam by drastically escalating the war. In each case he failed, and in so doing he imposed horrendous costs on his people and nation.